This invention relates to stirring molten metal and in a particular sense to procedure and apparatus for stirring metal such as aluminum in a furnace where the metal is melted, such stirring being effected for any of a variety of purposes, for example as to facilitate the melting of further portions of solid metal in an initial quantity of molten metal, or the mixing of added molten metal, or to effect incorporation of additions, e.g. other metals or the like for alloying, grain refining or similar functions, in an existing melt, or to maintain uniformity of composition or temperature in a standing body of molten metal.
One general type of furnace used for such melting operations with aluminum (herein understood to include aluminum alloys) embraces a horizontal vessel preferably of rectangular plan and commonly covered to provide a space wherein heat can be supplied by direct firing, i.e., with one or more fuel-burning nozzles directing flame across and downwardly toward the surface of the metal. Means are provided, as with doors in an upper part of a wall of the furnace, or a side well partitioned from the main chamber, for charging the furnace, and likewise means for tapping the melt, as by opening a conventional tap hole. In some cases, the furnace is arranged to be tilted, e.g. so that the metal can then run out through a spout, to be taken directly or indirectly to casting apparatus.
In these reverberatory and other types of melting furnace, it is desirable to stir the molten bath, e.g. to assist the melting operation, to reduce clustering of sludge on the furnace floor, to avoid inefficiency by losing heat from the surface without carrying it to lower levels of the melt, and especially to expedite dissolution of alloying additions, grain refiners and the like. A variety of methods have been used, including manual stirring by moving blades or like implements through the metal (causing turbulence, but little bulk flow), and different electromagnetic or analogous techniques. Among the latter are: induction stirring caused by external current paths, i.e., beneath the floor, stirring by magnetic means under the floor coacting with current, e.g. D.C., in the bath, and use of so-called jumping ring pumps placed in side wells to cause flow between the well and main chamber. Rotating mechanical paddles are also employed, for instance operated by an air motor; while this technique can induce major bulk flow by causing heavy local turbulence, it is not consistent with continuous use during firing.
The various electromagnetic methods can be designed to cause bulk flow and some local turbulence, but are apt to be expensive and difficult to embody with a furnace, or only partially effective.
There have been a number of other proposals, as for pumping molten metal between a melting chamber and a separate heating chamber, or in the case of some deep types of furnace or holding vessel, as for steel, by pumping the metal up to and through an upper vessel. In general, however, all of the prior methods have been less than fully satisfactory, for one or more of the reasons of cost of installation or operation, incomplete effectiveness in moving anything like all of the metal, availability only at special or limited times in the process of melting or holding the metal, and difficulty of construction in a way compatible with submergence in molten metal.
Of course, a very large variety of techniques have been employed or suggested for agitating liquids very different from molten metal, i.e., normal aqueous or other materials that are fluid at much lower temperatures, including the use of multiple stirring elements, or of vibrating means, or of means for moving liquid into and out of a large multiplicity of submerged apertures. It has not been at all feasible to use such methods for metal; indeed it has been apparent that complex structures or movable constructions cannot be achieved with heavy, brick-lined furnaces or with materials that will withstand the very high temperatures, the heavy mechanical loading, or the rapidly deteriorating effect of molten aluminum or other metal.
In consequence, there has remained a need for improvement in procedure or equipment for stirring large bodies of metal in furnaces, and at the same time there has been a lack of clear appreciation of some important advantages and economies that are attainable, as explained below, with good stirring operable throughout a large proportion of the time of using the furnace, whether for initial melting, dissolution of additions, or holding until or through successive tappings.